Information on the topography of a surface of an object is required in various areas of manufacturing. An area where the need for such information is particularly prominent is semiconductor manufacturing, where the semiconductor devices need to be inspected to ensure proper function. Such inspection includes specific structures making up the devices on a wafer, but also entities like solder bumps, which are required for holding components of a device together. For example, a die cut from a wafer may first be contacted to pins of a chip with an array of solder bumps. The chip can then be contacted to external circuitry by solder balls. For quality assurance, the heights of the solder bumps and solder balls with respect to the substrate have to be inspected before completion of the soldering.
Several methods for 3D topography measurements are well known in the art. Among these methods are white light interferometry, confocal microscopy, methods based on structured illumination, and laser triangulation with stereo vision. All these methods have their specific advantages and disadvantages.
White light interferometry is capable of providing height information of very high precision. The surface is moved in the interferometer by steps smaller than one wavelength; therefore, when inspecting semiconductor devices, a large number of frames of the surface needs to be taken and processed, as the steps have to extend over a range comparable with the height variation occurring on the surface.
Both confocal microscopy and methods based on structured illumination require rather standard microscope optics. Both approaches are better suited for inspecting surface topography at the scale of typical semiconductor devices. While confocal microscopy generally provides better height resolution than methods based on structured illumination, it also requires a more complicated and expensive optical setup.
The basic concept of methods based on structured illumination is to project a pattern, for example a grating, onto the surface of the object. There are two general approaches.
For an imaging system with low numerical aperture (NA), for example below 0.1, for which a longer working distance and a greater depth of focus are possible, the pattern can be projected onto the surface at an angle with respect to the imaging optical axis. Such an arrangement is similar to laser triangulation, as the fringe phase shift instead of the position shift of a line illumination is used to extract surface height. This approach is also known as phase shift fringe projection method.
In case of an imaging system with higher NA, above 0.1, neither oblique projection nor oblique imaging is easily implemented, as both depth of focus and working distance are limited. Here, instead, the pattern, for example a grating, is projected onto the surface through the imaging optics, and the optical axis of the imaging optics is normal to the surface of the object, more precisely to the plane defined by the general macroscopic extension of the surface. Due to this arrangement, height information cannot be extracted from fringe phase shift instead, height information can be obtained by moving the object in a direction parallel to the optical axis, and finding the position shift along this direction at which the contrast of the projected pattern is maximum.
There is a similarity between this setup and a confocal microscope, but the optics is simpler, not requiring relay optics. However, a higher data rate is required, as extracting the contrast of the pattern image requires three or more frames for each height position.
One example of such an approach, of structured illumination normal to the surface, can be found in U.S. Pat. No. 8,649,024 B2, issued on application Ser. No. 13/309,244. A pattern is generated by a spatial light modulator (SLM) and projected onto the surface of an object along an optical axis of an imaging objective. The object is moved relative to the objective along the optical axis, while the SLM modulates the projected pattern and a plurality of images are recorded. Maximum contrast of the projected pattern at a particular position on the surface yields height information for the respective position.
Which of the methods for 3D topography measurement mentioned above is best depends on the requirements of the specific measurement application. For semiconductor device inspection, some key requirements are: a resolution in the plane defined by the macroscopic extension of the surface of a few μm, a repeatability of positioning the object along a direction normal to this plane of less than 1 μm, a total range of movement along this normal direction of a few hundred μm. In view of this, methods based on structured illumination appear to be the most suitable for semiconductor device inspection by 3D topography measurements. The configurations of pertinent systems can cover a wide range both of resolution in the plane of the surface and of repeatability normal to the plane, and the methods achieve a large range of relative movement along the normal direction. The optics is comparatively simple and low cost, the setup of illumination and imaging along the normal direction is suitable for a wide variety of surface types, including both surfaces with predominantly specular reflection and surfaces with predominantly diffuse reflection. In particular with respect to the inspection of solder bumps, a larger NA yields a larger number of usable pixels at the spherical bump top of smaller bumps.
While the basic concept of structured illumination outlined above and exemplified in the cited U.S. Pat. No. 8,649,024 B2 achieves the required precision and accuracy, an unresolved problem is how to achieve these required characteristics while at the same time meeting ever increasing throughput requirements at preferably low cost, moreover in a manner that is scalable. For example, the spatial light modulator of the cited patent U.S. Pat. No. 8,649,024 B2 used for generating the patterned illumination is expensive, yet does not have the resolution and pixel counts for covering a large field of view, which, however, would be essential for higher throughput.